1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED), and more particularly, to an organic light emitting diode having improved luminous efficiency and lifespan.
2. Discussion of the Related Art
Although a cathode ray tube (CRT) was widely used as a display device, flat panel displays (FPDs), such as a plasma display panel (PDP), a liquid crystal display (LCD) and an organic light emitting diode (OLED) display device, have been recently researched and used. Among various FPDs, an OLED display device does not include a backlight unit required in an LED device having an emissive type because the OLED display device has a non-emissive type. As a result, the OLED display device has a light-weight and thin profile. In addition, the OLED display device has advantages in a viewing angle, a contrast ratio and power consumption as compared to the LCD device. Moreover, the OLED display device has a low direct current (DC) driving voltage and a fast response speed. Furthermore, since the inner components of the OLED display device are in a solid state, the OLED display device is resistant to an external impact and has a wide temperature range of usage. Specifically, since fabrication process is simple, the product cost of an OLED display device is less than that of an LCD device.
The OLED display device is an emissive type display device including an organic light emitting diode that emits a light through an organic emitting phenomenon. FIG. 1 is an energy band diagram showing an organic light emitting diode according to an related art. In FIG. 1, an organic light emitting diode 10 includes an anode 21, a cathode 25, a hole transporting layer (HTL) 33, an electron transporting layer (ETL) 35 and an emitting material layer (EML) 40. In addition, for the purpose of improving luminous efficiency, a hole injection layer (HIL) 37 is formed between the anode 21 and the HTL 33, and an electron injection layer (EIL) 39 is formed between the cathode 25 and the ETL 35.
When a high level voltage and a low level voltage are applied to the anode 21 and the cathode 25, respectively, of the organic EL diode 10, a hole of the anode 21 and an electron of the cathode 25 are transferred to the EML 40 to form an exciton. When the exciton transitions from an excited state to a ground state, a light is generated and the EML 40 emits the light.
The exciton may have a singlet state and a triplet state such that a ratio of existence probabilities of the singlet state and the triplet state is about 1:3. Since the OLED emits a light when the exciton transitions from the singlet state to the ground state, the OLED has a disadvantage that its internal quantum efficiency is not greater than about 25% theoretically. Accordingly, an OLED utilizing higher internal quantum efficiency of the triplet state (e.g. at least about 75%) would be desirable.